1. Field of the Invention
This invention relates to a process for synthesizing diamond by a combustion technique and more particularly to a process for synthesizing diamond by burning a hydrocarbon fuel gas and an oxidizing agent with a burner and directing the combustion flame against a surface of a substrate provided in face-to-face relation with the burner to form diamond on the substrate surface. The invention also relates to an synthesizing apparatus of diamond which is useful for synthesis of diamond.
2. Description of the Prior Art
Broadly, synthesizing methods of artificial diamond include a high pressure and high temperature synthesizing process and a low pressure vapor phase synthesizing process. In recent years, there has been proposed, as one of the lower pressure vapor phase synthesizing processes, a process of synthesizing diamond by a so-called combustion technique using a combustion flame. The working principal of this process is illustrated with reference to FIG. 2. As shown in the figure, hydrocarbon-based fuels such as, for example, acetylene, ethylene, methane, propane, methanol, ethanol and the like are provided as a carbon source for deposition of diamond. The fuel is subjected to combustion by means of a burner 11 along with pure oxygen serving as an oxidizing agent, which is used in an amount far smaller than the theoretical mixing ratio. By this, a flame 14 called feather is produced in a flame 12 obtained under such combustion conditions as mentioned above in the vicinity of a nozzle 13. When a substrate 15 is placed in the flame 14 and is forced to cool by a cooling means of a substrate holder 16 while keeping the substrate temperature at 300.degree. to 1200.degree. C., diamond is synthesized on the surface of the substrate 15.
Most of the diamond synthesizing processes hitherto proposed up to now are based on fundamental experiments wherein the burner 11 used in the synthesizing apparatus is, in most cases, commercially available cutting or welding torches. However, the injection port of the nozzle of these torches is a single port with a relatively small size. In this condition, even if the substrate 15 is optimumly provided, an area on which diamond is to be synthesized becomes very small is limited to 3 to 5 times the size of the injection port. The diameter of the injection port of commercially available burners is about 3 mm in maximum. Accordingly, the deposition area is only that of about 15 mm in diameter in maximum. Thus, the diamond synthesizing process using the combustion technique has the problem that when compared with chemical vapor deposition method (which may be hereinafter referred to as CVD methods) ordinarily used as one of diamond synthesizing processes, the area of synthesized diamond is smaller.
In general, cutting or welding torches have such a structure that a fuel and an oxidizing agent are mixed fairly upstream of an outlet of the injection port. In addition, the deposition time in combustion technique has to be continued over several hours and the substrate temperature has to be kept at high levels, so that the nozzle and its neighbours are exposed to a high temperature atmosphere and subjected to red heat, with a tendency toward back fire. This may lead to explosion accidents or damages by burning out of the torch.
On the other hand, in the above-described diamond synthesizing processes, it is one of very important factors for the synthesis of diamond to maintain the substrate 15 at a temperature of from 300.degree. to 1200.degree. C. The substrate is usually cooled either by a water-cooled pipe structure of the substrate holder 16 wherein water is passed through the pipe, or by a method wherein a cooling medium such as cooling water or a cooling gas is directly injected against the back of the substrate 15 mounted on the substrate holder 16. However, such a cooling structure or method is not effective in accurately controlling the temperature of the substrate 15, making it difficult to synthesize diamond with a good quality and/or a large size. Especially, with the latter cooling method, cooling water or a cooling gas is directly blown against the back of the substrate 15, so that the combustion flame is distrubed, with the problem that the diamond synthesis cannot be efficiently performed.
We made many attempts to synthesize diamond as one of studies of synthesizing diamond along with the development of diamond synthesizing apparatus. As a result, it was found that the diamond synthesizing process using the combustion technique is an economical method among low pressure vapor phase processes but it was very difficult to synthesize diamond with a large crystal size on the surface of the substrate. More particularly, the crystal growth is disturbed at the time when the crystal size reaches approximately 100 to 300 .mu.m. A longer deposition time results in commencement of deposition of graphite or carbon on the crystal surfaces. Thus, diamond with a larger crystal size cannot be synthesized.